Aluminum oxide carboxylate polymers are prepared by reacting aluminum alkoxides with fatty acids and occasionally with water. The compounds thus formed were characterized by the formula ##STR1## in the U.S. Pat. No. 2,744,074 (by Theobald); a similar structure, i.e. [0=Al-X].sub.n was proposed in the U.S. Pat. No. 2,925,430 by (Stedebauder and Viveen). In these formulae X means an organic acid radical and n is the degree of polymerization being at least 16. In the above cases a molecule of the formula ##STR2## wherein R stands for an alkyl group, is formed in the course of chain closure. According to our measurements concerning the number of alkoxy groups bound, n is higher than 25 in the above formula.
In the U.S. Pat. No. 2,979,497 Rimse characterized the aluminum oxide carboxylate polymers as a cyclic trimeric aluminum oxide form rendered probable by the very low alkoxy content of the pure compounds. However, this was not supported by any other measurement relating to the structure elucidation. The molecular weight measurement published in the U.S. Pat. No. 4,069,236 (by Hutchison) support the polymeric structure. Thus, the term "polymer" being more common in our opinion, is used in the present description.
In general, these compounds are used as gel-forming and gel-thickening agents for alkyd resins, glycerides, mineral oils and other hydrocarbons. Nowadays, their utilization is becoming increasingly widespread.
At present, the most important utilization of the aluminum complexes is in the production of fats. However, increasingly significant amounts are used in the printing, leather and cosmetic industries as well as in the production of fine chemicals. Aluminum complexes are suitable for several polymer compositions, as well as in the preparation of synthetic resins and plastics. The advantageous features of these compounds were shown in the Hungarian patent specification No. 192,257.
A known method of preparation of aluminum oxide carboxylate polymers proceeds via basic aluminum salts. According to the U.S. Pat. No. 2,925,430 basic aluminum salts of the formula (HO).sub.2 Al-X, wherein X stands for an organic acid radical, are reacted with alkoxides of the formula (RO).sub.2 Al-X, wherein R stands for a C.sub.1-10 alkyl group. In these reactions inert organic solvents are used and the alkanol liberated is distilled out of the system to obtain aluminum oxide carboxylate polymers of the formula ##STR3## wherein X, R and n are as defined above.
Another preparation method of aluminum carboxylate polymers is published in the French patent No. 1,555,831, in the U.S. Pat. No. 2,744,074 as well as in the British patent specification No. 806,113. In these processes the carboxylic acid used is added in a molar ratio higher than one in relation to the aluminum alkoxide. The carboxylic acid goes into reaction with the aluminum alkoxide, whereas an alkanol of the general formula R-OH, wherein R stands for an alkyl group, is formed in the reaction. When the carboxylic acid is added in a molar ratio exceeding one and the temperature of the reaction is increased to 120.degree. to 350.degree. C., then the aluminum compound is shifted towards the aluminum oxide carboxylate polymer and an ester is formed. The above process is illustrated by the following reaction equation: ##STR4## wherein R, X and n are as defined above.
A common feature of these processes is that in the course of the reaction a part of the carboxylic acid is utilized for ester formation and for the completion of polymerization at least a two-fold amount of the carboxylic acid is required. According to the references listed, molar excesses of 2.4 to 2.6 are commonly used.
According to a third method of the preparation of aluminum oxide carboxylates the aluminum alkoxide is brought into reaction with water and an organic carboxylic acid.
According to the British patent specification No. 825,878 aluminum alkoxide is dissolved in hot oil, then water dissolved in the alkanol is slowly added while maintaining the mixture near to the temperature of distilling off of the alkanol. After the addition the mixture is refluxed for several hours, then its temperature is increased to 140.degree. to 150.degree. C. whereby it becomes free from the alkanol. To the product thus obtained the corresponding carboxylic acid or carboxylic acid mixture is weighed at a lower temperature and after reaction occurs the mixture is freed from the solvent at a high temperature to obtain the aluminum oxide carboxylate polymers. The process described is complicated and tedious. During the mixing-in of the aqueous alkanol the precipitation of a solid-phase aluminum compound is observed which decreases the amount of aluminum compounds in the product and which can be separated only by filtration. In addition, the circuitous process favors ester formation and therefore even 15 to 25% by weight of ester can be present in the product which appears as a deficiency of the carboxylic acid in the polymer and can be corrected only by adding an additional amount of carboxylic acid. For this reason the carboxylic acid/aluminum mole ratio published in the examples reaches 1.7 instead of the theoretical value of 1.0. In the case of reactions carried out in our own experiments under similar conditions the above problems similarly occurred (Example 1).
According to the U.S. Pat. No. 2,979,497 aluminum alkoxide is dissolved in an aromatic solvent, then the alkanolic solution of the corresponding carboxylic acid and water is slowly added to the refluxing solution. After carrying out the reaction, the product is freed from alkanol at a high temperature of 140.degree. to 180.degree. C. In this process the precipitation of solid-phase aluminum compound cannot be avoided and the amount of ester formed as a side product reaches as high as 10% by weight.
The preparation of cyclic aluminum alkoxy and phenoxy compounds is described in the U.S. Pat. No. 3,054,816. Steam or a mixture containing steam diluted with alcoholic vapours are used to the condensation reaction of aluminum alkoxide. In the course of the reaction two moles of alkanol liberated in relation to one mole of water as well as the alkanol used for the dilution are continuously distilled off or are distilled off at the end of the reaction. On reacting an alkoxide in the vapor phase with steam, a solid product is obtained. Aromatic solvents or alkanols are employed as solvents for this reaction. This patent describes the preparation of cyclic aluminum oxide acetate carried out by adding acetic acid to the solution of cyclic aluminum oxide isopropoxide.
According to the U.S. Pat. No. 4,069,236 aluminum oxide carboxylate polymers are prepared from aluminum alkoxide by using an equimolar amount of water and a carboxylic acid is added at 130.degree. C. It is obvious from the examples that the precipitation of solid aluminum compounds cannot be eliminated. The product must be separated by filtration from the system. According to our own repeated experiments, the amount of ester formed in the above process exceeds 5% by weight.
The continuous preparation of cyclic aluminum oxide carboxylate trimers is described in the Hungarian patent No. 192,257. According to this process the reaction of aluminum oxide with water and carboxylic acids is simultaneously realized in a phase fluidized by an inert carrier gas. In comparison to the processes discussed above, this latter process is suitable for preparation of products with a relatively low proporation, e.g. 2 to 5% by weight, of ester. Due to the displacements of the addition ratios accidentally occurring during the continuous process the equimolar reaction of aluminum alkoxide with water and carboxylic acid is problematic since a gel formation can be started in the reactor which endangers the operation and the stability of the product.
A common feature of the processes of the prior art is that, in addition to the desired product, the formation of RX ester, wherein R is as defined above, cannot be eliminated or is even necessitated owing to the nature of these processes. J. Rinse (Paint Technology Vol. 28, No. 4) describes this reaction by the following scheme: EQU &gt;Al+XAl&lt;.fwdarw.&gt;Al--O--Al&lt;+RX
wherein R and X are as defined above.
This ester formation is particularly accelerated at higher temperatures, a fact supported also by our own experiments (see Example 4 compared to Example 3). This is the reason for the high yields of esters obtained according to the French patent No. 1,558,831, U.S. Pat. No. 2,744,074 and British patent specification No. 806,113. The significant ester formation should be taken in account at low temperatures too, when a reaction can freely proceed between the alkanol and carboxylic acid. It is likely that the latter reason plays a role in causing the amount of ester obtained as a side product in the processes discussed above to be high since these processes require long reaction times.
We have tried in our experiments to diminish the amount of ester, since when the carboxylic acid is used in an equimolar amount, the carboxylic group as functional group will partially be absent from the aluminum oxide carboxylate polymer product and the supplementation of it will necessitate an additional operation step lowering the facility of the process. Besides, the formation of an ester consuming carboxylic acid and alkanol represents an economic loss in itself.